This invention pertains to fuel cells and, in particular to a matrix member for retaining the electrolytes employed in these cells.
In recent years, a significant amount of effort has been expended in the development of commercially viable fuel cells. A fuel cell typically comprises two spaced electrodes between which is disposed an ion carrying electrolyte and to which are fed respective fuel and oxidant reactant gases. In a fuel cell of this type, proper selection of the ion carrying electrolyte is essential if the fuel cell is to operate satisfactorily. To this end, it has been found advantageous to employ strong acids such as, for example, H.sub.2 SO.sub.4 and H.sub.3 PO.sub.4, as the cell electrolyte, since these acids minimize power losses caused by polarization and electrolyte resistance. However, with such a liquid acid as the cell electrolyte, it becomes difficult to contain the electrolyte between the cell electrodes. Furthermore, while solid electrolytic members are available, these members generally have a greater resistance than the aforesaid liquid acid electrolytes. Thus, while these solid electrolytes can be more readily contained between the cell electrodes, their increased resistance prevents any significant improvement in cell operation over that achievable with liquid acid electrolytes.
In order to gain the high ionic conductivity benefit of liquid acid electrolytes, matrix members have been developed for compatibly supporting the liquid acid electrolytes. With such matrix members, the acid electrolyte is confined within the member and, hence, is prevented from flowing from between the cell electrodes. Additionally, the matrix member acts as a barrier to prevent crossover of the fuel and oxidant gases being fed to the electrodes.
U.S. Pat. No. 3,575,718 discloses one type of matrix member wherein the member comprises two adjacently arranged layers, each having a concentrated liquid acid immobilized therein and one of which borders the cell anode electrode and the other the cell cathode electrode. The layer bordering the anode electrode contains (by weight) from 5-90 percent carbon powder and from 5-90 percent fluorocarbon polymer gel. The carbon powder of this layer is stable in acids and has a surface area of at least about 1 square meter per gram, so as to permit the containment of an increased acid content. This increased acid content affords the layer and, hence, the matrix a high ionic or electrolytic conductivity. The layer bordering the cathode electrode, which is thinner of the two layers, in turn, comprises from 70-95 percent non-carbon inert inorganic compounds and from 5-90 percent fluorocarbon polymer gel. The inert inorganic compounds of this second layer have a low electronic conductivity and, thus, contribute this property to the matrix. The inert compounds also prevent matrix deformation, as well as promote liquid acid confinement.
U.S. Pat. Nos. 4,000,006 and 4,017,664 disclose a second type of matrix member wherein the matrix is in the form of a single layer comprised of at least 90 percent silicon carbide. The remainder of the layer is formed from a fluorocarbon polymer which acts as a binder for preventing shifting of the layer. In this matrix, the silicon carbide material itself affords the matrix the desired electrolytic, electronic and chemical stability properties.
It is an object of the present invention to provide an improved matrix member for use in supporting the liquid acid electrolyte of a fuel cell.
It is a further object of the present invention to provide a stable matrix member having high electrolytic conductivity and low electronic conductivity, while being capable of inhibiting reactant gas crossover at high pressures.
It is a further object of the present invention to provide a matrix of the last mentioned type having a bubble pressure approaching 50 psi.